Hidden consciousness, Sleep spindles and Recovery in Comatose Patients 

Coma is a deep unconsciousness state associated with brain failure. Comatose patients are behaviourally unresponsive. These disorders of consciousness are usually transitory but may last for an indefinite period at times. For clinical purposes, it is important to predict when such patients will regain consciousness and come out of coma.  

About 25% of unresponsive patients without an observable response to verbal commands or any detectable command-following behaviours show some degree of consciousness which is hidden to observers. When presented with cognitive tasks like motor imagery commands, the unresponsive patients with recent brain injuries show brain activity on functional magnetic resonance imaging (fMRI) or electroencephalography (EEG). This is the phenomenon of cognitive motor dissociation (CMD).  

It is known that comatose patients with cognitive motor dissociation (CMD) or hidden consciousness have better chances of achieving long-term recovery hence the imperative to identify unresponsive brain-injury patients with cognitive motor dissociation (CMD) or hidden consciousness.   

Recordings of EEG as the patient is presented with commands can be analysed to detect brain activity indicating hidden consciousness however implementing task-based EEG is difficult. Also, this gives false-negative results. EEG recording of brain waves associated with normal sleep patterns can be helpful because similar brain circuits between the thalamus and cortex (thalamocortical networks) are fundamental for both consciousness and for sleep control. Also, recording sleep brain waves is much easier and require no intervention. Further, sleep spindles (viz., brief bursts of brainwave activity during sleep) may reflect recovery of consciousness and cognitive function, as an earlier study suggests. The idea is to look for a less technically challenging complementary predictor of recovery which has been explored in a recent observational cohort study of 226 acutely brain injured patients. 

The researchers found that some patients showed very organised brief bursts of brainwave activity during sleep. On EEG graph, these bursts of electrical activity in brain are denoted as sleep spindles. Well-formed sleep spindles (WFSS) were observed in about 33% of the behaviourally unresponsive patients after acute brain injury. About half of patients with cognitive motor dissociation (CMD) showed sleep spindles which frequently preceded the detection of CMD. Also, patients with WFSS had a shorter time to recovery of consciousness suggesting sleep improvement may be helpful.  

Overall, both CMD and well-formed sleep spindles (WFSS) appeared to be associated with better chances of recovery of consciousness however about 14% patients did not show WFSS or CMD but recovered consciousness. So, although CMD and WFSS are predictors of recovery, they are not perfect predictors.  

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References:  

1. Bodien Y.G., et al 2024. Cognitive Motor Dissociation in Disorders of Consciousness. Published 14 August 2024. N Engl J Med 2024; 391:598-608. DOI: https://doi.org/10.1056/NEJMoa2400645 

2. Urakami Y. 2012. Relationship Between Sleep Spindles and Clinical Recovery in Patients With Traumatic Brain Injury: A Simultaneous EEG and MEG Study. Clinical EEG and Neuroscience. 2012;43(1):39-47. DOI: https://doi.org/10.1177/1550059411428718 

3. Carroll, E.E., Shen, Q., Kansara, V. et al. Sleep spindles as a predictor of cognitive motor dissociation and recovery of consciousness after acute brain injury. Nat Med (2025). Published: 03 March 2025. DOI: https://doi.org/10.1038/s41591-025-03578-x 

4. Columbia University. Research news – Sleep Patterns May Reveal Comatose Patients with Hidden Consciousness. 3 March 2025. Available at https://www.cuimc.columbia.edu/news/sleep-patterns-may-reveal-comatose-patients-hidden-consciousness  

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